I don't understand this python __del__ behaviour

Question

Can someone explain why the following code behaves the way it does:

import types

class Dummy():
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print "delete",self.name

d1 = Dummy("d1")
del d1
d1 = None
print "after d1"

d2 = Dummy("d2")
def func(self):
    print "func called"
d2.func = types.MethodType(func, d2)
d2.func()
del d2
d2 = None
print "after d2"

d3 = Dummy("d3")
def func(self):
    print "func called"
d3.func = types.MethodType(func, d3)
d3.func()
d3.func = None
del d3
d3 = None
print "after d3"

The output (note that the destructor for d2 is never called) is this (python 2.7)

delete d1
after d1
func called
after d2
func called
delete d3
after d3

Is there a way to "fix" the code so the destructor is called without deleting the method added? I mean, the best place to put the d2.func = None would be in the destructor!

Thanks

[edit] Based on the first few answers, I'd like to clarify that I'm not asking about the merits (or lack thereof) of using __del__. I tried to create the shortest function that would demonstrate what I consider to be non-intuitive behavior. I'm assuming a circular reference has been created, but I'm not sure why. If possible, I'd like to know how to avoid the circular reference....

Answer 1

You cannot assume that __del__ will ever be called - it is not a place to hope that resources are automagically deallocated. If you want to make sure that a (non-memory) resource is released, you should make a release() or similar method and then call that explicitly (or use it in a context manager as pointed out by Thanatos in comments below).

At the very least you should read the __del__ documentation very closely, and then you should probably not try to use __del__. (Also refer to the gc.garbage documentation for other bad things about __del__)

Answer 2

I'm providing my own answer because, while I appreciate the advice to avoid __del__, my question was how to get it to work properly for the code sample provided.

Short version: The following code uses weakref to avoid the circular reference. I thought I'd tried this before posting the question, but I guess I must have done something wrong.

import types, weakref

class Dummy():
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print "delete",self.name

d2 = Dummy("d2")
def func(self):
    print "func called"
d2.func = types.MethodType(func, weakref.ref(d2)) #This works
#d2.func = func.__get__(weakref.ref(d2), Dummy) #This works too
d2.func()
del d2
d2 = None
print "after d2"

Longer version: When I posted the question, I did search for similar questions. I know you can use with instead, and that the prevailing sentiment is that __del__ is BAD.

Using with makes sense, but only in certain situations. Opening a file, reading it, and closing it is a good example where with is a perfectly good solution. You've gone a specific block of code where the object is needed, and you want to clean up the object and the end of the block.

A database connection seems to be used often as an example that doesn't work well using with, since you usually need to leave the section of code that creates the connection and have the connection closed in a more event-driven (rather than sequential) timeframe.

If with is not the right solution, I see two alternatives:

1. You make sure __del__ works (see this blog for a better description of weakref usage)
2. You use the atexit module to run a callback when your program closes. See this topic for example.

While I tried to provide simplified code, my real problem is more event-driven, so with is not an appropriate solution (with is fine for the simplified code). I also wanted to avoid atexit, as my program can be long-running, and I want to be able to perform the cleanup as soon as possible.

So, in this specific case, I find it to be the best solution to use weakref and prevent circular references that would prevent __del__ from working.

This may be an exception to the rule, but there are use-cases where using weakref and __del__ is the right implementation, IMHO.

Answer 3

Instead of del, you can use the with operator.

http://effbot.org/zone/python-with-statement.htm

just like with filetype objects, you could something like

with Dummy('d1') as d:
    #stuff
#d's __exit__ method is guaranteed to have been called

Answer 4

del doesn't call __del__

del in the way you are using removes a local variable. __del__ is called when the object is destroyed. Python as a language makes no guarantees as to when it will destroy an object.

CPython as the most common implementation of Python, uses reference counting. As a result del will often work as you expect. However it will not work in the case that you have a reference cycle.

d3 -> d3.func -> d3

Python doesn't detect this and so won't clean it up right away. And its not just reference cycles. If an exception is throw you probably want to still call your destructor. However, Python will typically hold onto to the local variables as part of its traceback.

The solution is not to depend on the __del__ method. Rather, use a context manager.

class Dummy:
   def __enter__(self):
       return self

   def __exit__(self, type, value, traceback):
       print "Destroying", self

with Dummy() as dummy:
    # Do whatever you want with dummy in here
# __exit__ will be called before you get here

This is guaranteed to work, and you can even check the parameters to see whether you are handling an exception and do something different in that case.

Answer 5

It seems to me the real heart of the matter is here:

adding the functions is dynamic (at runtime) and not known in advance

I sense that what you are really after is a flexible way to bind different functionality to an object representing program state, also known as polymorphism. Python does that quite well, not by attaching/detaching methods, but by instantiating different classes. I suggest you look again at your class organization. Perhaps you need to separate a core, persistent data object from transient state objects. Use the has-a paradigm rather than is-a: each time state changes, you either wrap the core data in a state object, or you assign the new state object to an attribute of the core.

If you're sure you can't use that kind of pythonic OOP, you could still work around your problem another way by defining all your functions in the class to begin with and subsequently binding them to additional instance attributes (unless you're compiling these functions on the fly from user input):

class LongRunning(object):
    def bark_loudly(self):
        print("WOOF WOOF")

    def bark_softly(self):
        print("woof woof")


while True:
    d = LongRunning()
    d.bark = d.bark_loudly
    d.bark()

    d.bark = d.bark_softly
    d.bark()

Answer 6

A full example of a context manager.

class Dummy(object):
    def __init__(self, name):
        self.name = name
    def __enter__(self):
        return self
    def __exit__(self, exct_type, exce_value, traceback):
        print 'cleanup:', d
    def __repr__(self):
        return 'Dummy(%r)' % (self.name,)

with Dummy("foo") as d:
    print 'using:', d

print 'later:', d

Answer 7

An alternative solution to using weakref is to dynamically bind the function to the instance only when it is called by overriding __getattr__ or __getattribute__ on the class to return func.__get__(self, type(self)) instead of just func for functions bound to the instance. This is how functions defined on the class behave. Unfortunately (for some use cases) python doesn't perform the same logic for functions attached to the instance itself, but you can modify it to do this. I've had similar problems with descriptors bound to instances. Performance here probably isn't as good as using weakref, but it is an option that will work transparently for any dynamically assigned function with the use of only python builtins.

If you find yourself doing this often, you might want a custom metaclass that does dynamic binding of instance-level functions.

Another alternative is to add the function directly to the class, which will then properly perform the binding when it's called. For a lot of use cases, this would have some headaches involved: namely, properly namespacing the functions so they don't collide. The instance id could be used for this, though, since the id in cPython isn't guaranteed unique over the life of the program, you'd need to ponder this a bit to make sure it works for your use case... in particular, you probably need to make sure you delete the class function when an object goes out of scope, and thus its id/memory address is available again. __del__ is perfect for this :). Alternatively, you could clear out all methods namespaced to the instance on object creation (in __init__ or __new__).

Another alternative (rather than messing with python magic methods) is to explicitly add a method for calling your dynamically bound functions. This has the downside that your users can't call your function using normal python syntax:

class MyClass(object):
    def dynamic_func(self, func_name):
        return getattr(self, func_name).__get__(self, type(self))

    def call_dynamic_func(self, func_name, *args, **kwargs):
        return getattr(self, func_name).__get__(self, type(self))(*args, **kwargs)

    """
    Alternate without using descriptor functionality:
    def call_dynamic_func(self, func_name, *args, **kwargs):
        return getattr(self, func_name)(self, *args, **kwargs)
    """

Just to make this post complete, I'll show your weakref option as well:

import weakref
inst = MyClass()
def func(self):
    print 'My func'
#  You could also use the types modules, but the descriptor method is cleaner IMO
inst.func = func.__get__(weakref.ref(inst), type(inst))

Answer 8

use eval()

In [1]: int('25.0')                                                                                                                                                                                                                                                               
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-67d52e3d0c17> in <module>
----> 1 int('25.0')

ValueError: invalid literal for int() with base 10: '25.0'

In [2]: int(float('25.0'))                                                                                                                                                                                                                                                        
Out[2]: 25

In [3]: eval('25.0')                                                                                                                                                                                                                                                              
Out[3]: 25.0